1. Field of the Invention
This invention relates to a four-wheel steering gear provided with a plurality of hydraulic systems having control valves, in which upon a failure of a main hydraulic system, another hydraulic system is used for controlling an actuator, and more particularly, to a fail-safe circuit for use therein.
2. Description of Related Art
There is a system of connecting a hydraulic pump to an actuator, such as in a four-wheel steering gear, through a control valve so as to control the driving of the actuator. In this case, the control system controls the control valve to control the direction and amount of the flow of pressure oil so as to apply the pressure oil to the actuator.
In such systems, if a control valve is not controlled properly due to a failure of a control system, for example, a circuit between the control system and a control valve is interrupted in order that an actuator is not controlled by an error control signal outputted from the control system.
When the control system has some problems, if the driving of the actuator is stopped immediately, this often causes inconveniences. As shown in FIG. 2, therefore, a system having a dual-redundant control drive has been considered.
In the system, an actuator 1 is connected to two control circuits: a main circuit connecting between the actuator 1 and a main pump 2 through a first control valve 3, and a sub circuit connecting the actuator 1 and a sub pump 5 through a second control valve 6.
The first control valve 3 is provided with a supply port 3a connected to the main pump 2, and a return port 3b connected to a tank T, and also is a proportioning solenoid valve provided with actuator ports 3c and 3d which serves as a supply port or a return port for the actuator 1. Switched positions of the first control valve 3 are controlled by a main CPU 4.
The aforementioned second control valve 6 is also of a proportioning solenoid valve controlled by a sub CPU 7, which is provided with a supply port 6a, return port 6b and actuator ports 6c and 6d as in the case of the first control valve 3.
The second control valve 6 is usually maintained at a neutral position and the first control valve 3 is switched to control the actuator. When the main CPU 4 cannot properly control the first control valve 3 due to a failure of the main CPU 4, the first control valve 3 returns to a neutral position and the sub CPU 7 controls the second control valve 6 to control the driving of actuator 1.
The above conventional system shown FIG. 2 includes the two hydraulic pumps in order that the sub hydraulic circuit is triggered for the operation of the actuator when the main hydraulic system encounters abnormal conditions. The two pumps need a large installation area and costs.
In the case of providing one hydraulic pump and a changeover valve for switching flowing of pressure oil between the main circuit and the sub circuit, the circuits become complicated. Moreover, if the changeover valve suffers a failure, it may be impossible to use the sub circuit provided especially.
Therefore, it is an object of the present invention to provide a fail-safe circuit designed to allow a sub circuit to reliably operate when a main circuit encounters abnormal conditions even in a supply source of pressure oil.
A first aspect of the present invention is a fail-safe circuit characterized by including: a supply source of pressure oil; an actuator; a plurality of solenoid type control valves provided between the supply source and the actuator and connected to the actuator in parallel, and in that the control valve is provided with a centering spring to maintain at a neutral position in normal times, and with a supply port, a return port and two actuator ports, configured to communicate the supply port and the return port with each other and not to communicate the supply port and the return port with the respective actuator ports at the neutral position, and to connect a supply port of a control valve located at the most upstream end of the control valves to the supply source, and to connect a return port of a control valve located at the most down stream end of the control valves to a tank while the each return port of the control valves is connected to a supply port of a control valve located downstream from and adjacent to the connected return port, and in that the driving of the actuator is controlled while only a specified control valve of the plurality of control valves is switched and other control valves are maintained at the neutral position.
According to the first invention, if one control system encounters abnormal conditions, only by cutting off the failed control system, another control system is triggered.
Moreover, since one pressure oil source constitutes a plurality of control systems, the economics are improved.
A second aspect of the present invention, predicated on the first aspect, is characterized in that the plurality of control valves comprises a first control valve and a second control valve, and characterized by further including a main CPU for controlling the first control valve and a sub CPU for controlling the second control valve, the main CPU stopping the controlling operation for the first control valve when the main CPU itself encounters abnormal conditions, and the sub CPU detecting the abnormal conditions in the main CPU and controlling the driving of the second control valve.
According to the second invention, if the main CPU encounters abnormal conditions, it is possible that the sub CPU controls the second control valve to continue the control for driving the actuator.
A third aspect of the present invention, predicated on the first and second aspects, is characterized in that the plurality of control valves comprises a first control valve and a second control valve, and characterized by further including a main CPU for controlling the first control valve and a sub CPU for controlling the second control valve, in which the main CPU stops the controlling operation for the first control valve when detecting abnormal conditions in the first control valve, and the sub CPU detects a halt on the controlling operation of the main CPU and controls the driving of the second control valve.
According to the third invention, if the first control valve encounters abnormal conditions even though the main CPU operates normally, it is possible that the main CPU stops the controlling operation and the sub CPU starts the controlling operation for the second control valve to continue the control for driving the actuator.
A fourth aspect of the present invention is characterized by a cut-off valve connected upstream or downstream from the control valve.
According to the fourth invention, if the control valve fails mechanically and does not return to the neutral position, it is possible that the cut-off valve cuts off the communication between the actuator port of the failing control valve and the actuator.